Basically, this law states that if you push on the computer screen, the screen is going to exert the same force back on your hand. The force the screen exerts is called the reaction. That's what you feel when you push on something. To go back to the falling computer screen mentioned earlier, there is a force exerted on both the ground and the screen, but the screen gets affected much more noticeably.
There is also another law from Isaac Newton, this one very important for Statics.
Now that you know what a force is, how do you represent it? There are three important things to know: where, how much, and in what direction. The "Where" is called the point of application and specifies the place that the force is directed upon. If you're pushing on the computer screen, the point of application is the part of the screen covered by your fingertip.
"How Much" is called the magnitude of the force and is expressed in a unit called a Newton (named in honor of, you guessed it, Isaac Newton), abbreviated as "N". This is the International System unit, basically the metric system. If you insist on knowing, the Pound is the unit used in the US, though it's pretty outdated and we'll only be using the Newton in the rest of this web site. Technically, the Newton is defined as the force required to accelerate one kilogram of material at 1 meter per second per second. The two "per second"s weren't a mistake, that's how you represent acceleration. Each second, the mass goes one meter per second faster than it was going before.
The direction of a force can easily be represented by an arrow pointing in the direction that it is going. This is the simplest representation and is very useful when drawing a picture, such as the one on the right, to try to visualize how an object is being affected. Sometimes, it is also useful to make the length of the force's arrow proportional to it's magnitude. The width of an arrow doesn't represent anything, in case you were wondering.